(1) Field of the Invention
The present invention relates to perspective distortion correction performed on an image captured by a monitoring apparatus and so on.
(2) Description of the Related Art
Conventionally, solid-state imaging elements such as a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS) have been used for monitoring apparatuses. Widely used as such monitoring apparatuses is a monitoring apparatus which performs image processing on a captured image so as to detect an object, particularly, a person. As a method of detecting a person in the image, a method using a pattern recognition technique based on statistical learning as disclosed in Non-Patent Reference 1 is generally used (Dalai, N. and Triggs, B., “Histograms of oriented gradients for human detection”, Computer Vision and Pattern Recognition, 2005. IEEE Computer Society Conference, pages: 886-893).
In this method, each of the following techniques is used. A feature selection technique is used for extracting, from the captured image, a feature quantity used for identification. An identifier construction technique is used for constructing an identifier which takes the selected feature quantity as input and determines whether or not the selected feature quantity represents the target. A technique to determine, using the constructed identifier, whether or not there is a person in a partial area in the imaging area is used.
A monitoring apparatus using a normal lens is provided at a constant angle of depression in a high position in a ceiling and so on. Thus, in the image captured from above, a significant perspective distortion is caused as a result of projecting a three-dimensional subject onto a two-dimensional imaging element. This causes a difference in tilt of a person in an image portion, even if it is the image portion of the same person standing upright, depending on whether the image portion of the person is located in a center of the image or at an end portion. In other words, this causes a difference in direction of the person (a direction from a foot to a head, and so on).
However, the identifier constructed for human detection is only capable of detecting a person having a particular tilt in the image. Therefore, to automatically detect all people in the image captured by the monitoring apparatus, a method generally used is to generate identifiers corresponding to tilts of all the people that can be captured in the image and perform judgment processing the number of times equivalent to the number of identifiers.
In addition, in the monitoring apparatus, to respond to the needs for monitoring an entire range with one camera, an omnidirectional imaging apparatus using a fisheye lens, an omnidirectional mirror, and so on has come to be used more often than before. In the case of the monitoring apparatus using the omnidirectional imaging apparatus, a special circular image is captured, so that the detection is often performed on an image generated by converting the captured image.
For example, in Non-Patent Reference 2 (Tatsuo Sato, Kazuhiro Goto, “Extension Method for Omni-directional Camera”, OITA-AIST Joint Research Center, Industrial Technology Div., Vol: 2002, pages 9 to 11), panoramic extension is performed on an omnidirectional image which maintains an aspect ratio of an object, based on a center of the circular image. As a result, this equalizes the directions of the upper and lower parts of the person in the extended image, thus allowing the detection to be performed using the same processing as the processing for an image captured with a normal lens.